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Kepler-78b is a planet that shouldn’t exist. This scorching lava world circles its star every eight and a half hours at a distance of less than one million miles — one of the tightest known orbits. According to current theories of planet formation, it couldn’t have formed so close to its star, nor could it have moved there.

Kepler-78b is a planet that shouldn’t exist. This scorching lava world, shown here in an artist’s conception, circles its star every eight and a half hours at a distance of less than one million miles. According to current theories of planet formation, it couldn’t have formed so close to its star, nor could it have moved there. (Credit: David A. Aguilar (CfA))

“This planet is a complete mystery,” says astronomer David Latham of the Harvard-Smithsonian Center for Astrophysics (CfA). “We don’t know how it formed or how it got to where it is today. What we do know is that it’s not going to last forever.”

“Kepler-78b is going to end up in the star very soon, astronomically speaking,” agrees CfA astronomer Dimitar Sasselov.

Not only is Kepler-78b a mystery world, it is the first known Earth-sized planet with an Earth-like density. Kepler-78b is about 20 percent larger than Earth, with a diameter of 9,200 miles, and weighs almost twice as much. As a result it has a density similar to Earth’s, which suggests an Earth-like composition of iron and rock.

ScienceDaily (Aug. 28, 2012) — Coming less than a year after the announcement of the first circumbinary planet, Kepler-16b, NASA’s Kepler mission has discovered multiple transiting planets orbiting two suns for the first time. This system, known as a circumbinary planetary system, is 4,900 light-years from Earth in the constellation Cygnus.

Orbiting in the Habitable Zone of Two Suns: This diagram compares our own solar system to Kepler-47, a double-star system containing two planets, one orbiting in the so-called “habitable zone.” (Credit: NASA/JPL-Caltech/T. Pyle)

This discovery proves that more than one planet can form and persist in the stressful realm of a binary star and demonstrates the diversity of planetary systems in our galaxy.

Astronomers detected two planets in the Kepler-47 system, a pair of orbiting stars that eclipse each other every 7.5 days from our vantage point on Earth. One star is similar to the sun in size, but only 84 percent as bright. The second star is diminutive, measuring only one-third the size of the sun and less than 1 percent as bright.

ScienceDaily (June 21, 2012) — One is a rocky planet 1.5 times the size of Earth. The other is a gaseous world nearly four times Earth’s size. Together they form a spectacular system in which two planets orbit closer to each other than any yet discovered

“We’ve never known of planets like this,” said Yale University astronomer Sarbani Basu, a member of the research team that analyzed the system. “If you were on the smaller planet looking up, the larger planet would seem more than twice the size of Earth’s full moon. It would be jaw-dropping.”

Basu’s research focused on determining the properties of the planets’ host star — work that was essential for discerning the characteristics of the orbiting planets.

The 46-member, international team, led by astronomers at Harvard and the University of Washington, report their discovery June 21 in Science Express, the early release version of the journal Science.